Noninvasive in vivo fluorescence measurement of airway-surface liquid depth, salt concentration, and pH

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Abstract

The concentration of salt in the thin layer of fluid at the surface of large airways, the airway-surface liquid (ASL), is believed to be of central importance in airway physiology and in the pathophysiology of cystic fibrosis. Invasive sampling methods have yielded a wide range of ASL [NaCl] from 40 to 180 mM. We have developed novel fluorescent probes and microscopy methods to measure ASL thickness, salt concentration, and pH quantitatively in cell-culture models and in the trachea in vivo. By rapid z-scanning confocal microscopy, ASL thickness was 21 ± 4 μm in well-differentiated cultures of bovine tracheal epithelial cells grown on porous supports at an air-liquid interface. By ratio imaging fluorescence microscopy using sodium, chloride, and pH-sensitive fluorescent indicators, ASL [Na+] was 97 ± 5 mM, [Cl–] was 118 ± 3 mM, and pH was 6.94 ± 0.03. In anesthetized mice in which a transparent window was created in the trachea, ASL thickness was 45 ± 5 μm, [Na+] was 115 ± 4 mM, [Cl–] was 140 ± 5 mM, and pH was 6.95 ± 0.05. Similar ASL tonicity and pH were found in cystic fibrosis (CFTR-null) mice. In freshly harvested human bronchi, ASL thickness was 55 ± 5 μm, [Na+] was 103 ± 3 mM, [Cl–] was 92 ± 4 mM, and pH was 6.78 ± 0.2. These results establish by a noninvasive approach the key properties of the ASL and provide direct evidence that the ASL is approximately isotonic and not saltier in cystic fibrosis.

Authors

Sujatha Jayaraman, Yuanlin Song, L. Vetrivel, Leena Shankar, A.S. Verkman